No, they aren't - they are special constructs inside s:decorate.
There is an open JIRA issue for the ability to customise whether the div is output or not. Vote for it!
I found several issues pertaining to s:decorate, including one for allowing the developer to style the outputted <div>, but I could not find the particular issue you speak of. I would love to vote for it!
In fact, I would love to see this capability added. Do you know if this going to be happening anytime soon?
Would you recommend that we make the modification ourself? We're really hoping to use s:decorate without the <div>.
Thanks for your time.